R-f window for high power electron tubes



June 3, 1969 PRElST ET AL RF WINDOW FOR HIGH POWER ELECTRON TUBESOriginal Filed April 16, 1962 UNCOATED-V,

INVENTORS DONALD H. PREIST RUTH CARLSON mLcorr BY Mt%8 Z22 ATTORNEYUnited States Patent US. Cl. 333-98 4 Claims ABSTRACT OF THE DISCLOSUREFailure of windows transparent to radio-frequency electromagnetic energydue to single surface and two surface multipactoring of electronsinvolving the window is described. Window structures including means forpreventingsuch failure are disclosed and the manufacture of suchstructures and their incorporation into a radio frequency device,specifically a klystron, are described.

This application is a division of our co-pending application Ser. No.187,521 filed Apr. 16, 1962, now Patent No. 3,252,034.

This invention relates to windows transparent to radiofrequencyelectromagnetic energy, and particularly to such windows fabricated in amanner to render them less susceptible to heating due to electronbombardment.

..In the electron tube industry, one of the limiting factors determiningthe amount of ower which can be extracted from an electron tube involvesthe output means for the tube. The output means is often in the form ofdielectric material forming a window transparent to radio-frequencyelectromagnetic energy. The limiting factor is that the amount of powerwhich a window is capable of passing has not kept pace with the amountof power which an electron tube is capable of generating. Thus, while itis possible with state-of-the-art electron tubes to generate very highpowers, it is becoming increasingly difiicult to extract such power fromthe electron tube because of the limitations imposed by the outputmeans, especially where such output means is a dielectric window as usedin micro-wave tubes such as klystrons, for instance.

I Output windows transparent to electromagnetic energy are usuallyfabricated from dielectric materials having a coefiicient of secondaryelectron emission greater than unity for values of an acceleratingelectric field greater than about.40-8O volts, and this characteristicrenders them susceptible to destructive heating due to single surfacemultipactoring by secondary electrons emitted from the window itselfwhen the accelerating electric field exceeds a critical value. Thisphenomenon i explained at length in an article by the inventors herein,appearing in the IRE Transactions of the Professional Group on ElectronDevices, vol. EE8, No. 4, dated July 1961. It is thereforeone of theobjects of the invention to provide a composition of dielectrim materialwhich possesses a maximum coeflicient of secondary emission of electronsof about unity or less for values of bombarding electron velocity atleast several times higher than the usual range of 40-80 volts.

. Another object of the invention is to provide an article ofmanufacture fabricated from a dielectric material having a maximumcoeflicient of secondary electron emission of not substantially morethan unity under the conditions stated in the preceding paragraph.

Another of the important objects of the invention is to provide aradio-frequency output window fabricated from a material having asecondary electron emission coeflicient of about or less than unity inthe presence of free 3,448,413 Patented June 3, 1969 electrons in vacuumand bombarding electron velocity substantially higher than 4080 volts.

Still another object of the invention is the provision of an outputwindow having a coeflicient of secondary electron emission of about orless than unity and which also possesses thermal stress resistancesufficient to withstand temperature gradients caused by dielectriclosses, for example, without failure due to fracture, softening, orchemical instability usually associated with such temperature gradients.

Still another important object of the invention is the provision ofradio-frequency output window having a coefiicient of secondary electronemission of about or less than unity, and which also possesses therequisite dielectric strength even at elevated temperatures to withstandvery high voltage gradients.

A still further object of the invention is the provision of a high powerradio-frequency output window for electron tubes which possesseschemical stability even when subjected to extremes in temperature suchas during bakeout.

Still another object of the invention is the provision of a microwavestructure incorporating such a window.

A still further object of the invention is the provision of a dielectricand metallic microwave structure incorporating a dielectricradio-frequency window fabricated from material treated to prossess asecondary electron emission coefiicient of about or less than unity invacuum and in the presence of free electrons, and in which the metallicparts of the structure have also been treated so as to have a secondaryelectron emission coefficient of about or less than unity.

A still further object of the invention is to provide an electron tubehaving at least one resonant cavity portion equipped with at least oneradio-frequency window which possesses a secondary electron emissioncoeflicient of about or less than unity, and in which the conductivemetallic walls of the resonant cavity also possess a secondary electronemission coefficient of about or less than unity.

The power-passing limitations which afiiict radio-frequency windows areapplicable alike to cylindrical and fiat resonant cavity Windows andalso to fiat and conical windows such as are used in waveguides. It istherefore another object of the invention to provide a microwave windowassembly constructed in a manner to decrease electron bombardment of thewindow and thereby decrease the amount of heat generated in the window,and to dissipate by efiicient conduction such heat as is generated inthe window.

A still further object of the invention is the provision of a resonantcavity portion for a klystron tube in which the window is fabricatedfrom a high thermal conductivity dielectric such as beryllia, and theassociated metallic parts are fabricated from oxygen-freehigh-conductivity copper, with both the dielectric and metal portions ofthe cavity coated with a layer of metallic material which reduces toabout or less than unity the coefficient of secondary electron emissionfrom both the dielectric and metal parts.

The invention possesses other object and features of value, some ofwhich, with the foregoing, will be apparent from the followingdescription and the drawings. It is to be understood, however, that theinvention is not limited to the embodiments illustrated and described,but may be incorporated in varying forms within the scope of theappended claims.

Broadly considered, the invention in one of its aspects coprises theprovision of a dielectric material having a coefi'cient of secondaryelectron emission of about or less than unity for values of bombardingelectron velocity at least several times higher than the usual range of4080 volts; and in another aspect the utilization of such material incombination with metallic bodies or surfaces which also have a secondaryelectron emission coefiicient of about or less than unity to provideradio-frequency high-power output windows for microwave devices. It iswell known in the art that dielectric materials suitable as high poweroutput windows normally have, in the presence of charged particles, suchas electrons, and high radio-frequency alternating electric fields, acoefficient of secondary electron emission substantially greater thanunity. For alumina ceramics, for instance, the maximum secondary ratiois greater than 3. According to this invention the dielectric windowmaterial has been rendered less susceptible to secondary electronemission over a wider hange of bombarding velocities by the applicationthereto of titanium by a method described and claimed in a copendingapplication. The titainum is preferably incorporated in the dielectricmaterial after the dielectric material has been fabricated into thedesired configuration for use as an output window, but it is believedthat such incorporation may also be effected during the manufacture ofthe dielectric material. In the former case, it has been found thattitanium deposited on selected dielectric materials by vacuum depositionto a thickness of about 100 Angstrom units or less will result in adielectric composition which retains the requisite dielectric strengthto be used as a high-frequency, high-power output window, together withthe other requisite characteristics pertaining to dielectric loss,thermal stress, and chemical stability, as discussed above. Whenincorporated in a metallic structure such as a resonant cavity orwaveguide, the metallic structure is also provided with a layer oftitanium, so that the secondary electron emission coefiicinet of themetallic surfaces will also be about or less than unity. Such a windowconstruction has been found to be completely free from destructiveheating due to secondary emission of electrons from the surface of thewindow itself, or from secondarily emitted electrons originating in theadjacent metallic structure.

Referring to the drawing:

FIGURE 1 is an elevation of a klystron tube incorporating aradio-frequency output window in accordance with the invention. Theoutput window assembly is shown in vertical section.

FIGURE 2 is an enlarged vertical sectional view of that portion ofFIGURE 1 indicated by the bracket 2. The titanium coatings on theinterior surfaces are shown in exaggerated thickness for clarity.

FIGURE 3 is vertical sectional view through a waveguide portion equippedwith a window according to this invention.

FIGURE 4 is a graph showing the secondary emission coetficient (3) as afunction of velocity (V) of bombarding electrons expressed in volts forcoated and uncoated dielectric windows.

In the early days of UHF, when klystron tubes, for instance, were firstcoming into their own, radio-frequency transparent windows were almostexclusively made of glass, and were capable of handling only a fewkilowatts of CW power. In such early klystron tubes, power output waslimited by failure of the window in the output cavity, and such failurewas ascribed to overheating of the glass window caused by dielectriclosses in the window material. As the industry became moreknowledgeable, ceramic materials having greater resistance to thermalstresses were substituted for glass materials. The geometries orconfigurations of associated metallic structures also changed in orderto take advantage of ceramic materials. Ceramic windows enable reliableoperation at the IO-kilowatt level at frequencies between 700 and 1,000megacycles, and considerably higher power at lower frequencies. In therace to develop tubes of the klystron type having higher frequencies andhigher output power, the assignee of the present invention pioneered theutilization of external resonant cavities coupled to an electromagneticfield within the klystron envelope through a dielectric or ceramicwindow. A klystron tube of this type is illustrated, described andclaimed in US. Patent No. 2,619,611, issued to the assignee of thepresent invention. As the power output increased from these new andbetter tubes, the limiting factor again became the tendency of the largeoutput windows to rupture, thus destroying the envelope. Even integralcavity tubes, which utilized dielectric or ceramic windows interposed ina waveguide output, suffer from the inability of passing through thewaveguide window the large power output from these tubes.

It was believed at the outset that the dielectric losses of thedielectric material itself, usually ceramic, caused heating of theceramic material which resulted in rupture thereof. Such dielectriclosses do of course cause heating, but usually not sufiicient heating torupture the window. Another theory that has been advanced to explain therupturing of windows at high power outputs is that electrons from theelectron beam or other source bombard the window and cause rupture. Astill further theory is that secondary electrons, released from themetallic output gap drift tube assemblies by bombarding primaries of thebeam, arrive at the window with high velocity,'the kinetic energy of theelectrons being transformed into heat on the window, and the windowbeing caused to rupture as a result hereof. It has been found that undersome special circumstances excessive multipactoring at the output gapcan cause such bombardment of the window, but in the usual case does notoccur.

Through an extensive study program it has been determined that in theusual case heating and resultant rupture of dielectric output windows iscaused by a phenomenon known as single surface multipactoring on thesurface of the dielectric material itself, triggered by an electron orlectrons arriving at the window from within the envelope and impingingon the window with a force at least sufficient to librate secondaryelectrons from the dielectric material. The phenomenon is calledtwo-surface multipactoring when the multipactoring occurs between awaveguide surface and the adjacent surface of an inclined waveguidewindow, or between the diverging surfaces adjacent the apex end of aconical waveguide window. The triggering electrons do not usually comefrom the beam, but rather are emitted from adjacent metal parts as aresult of the presence of high intensity electric fields. It has beenfound that the free secondary electrons liberated from the dielectricwindow material are accelerated to critical velocities by thealternating electric field adjacent the vacuum side of the window andare redirected toward the dielectric window such that additional freesecondaries are liberated from the dielectric.

material when the accelerated and redirected free secondary electronsimpinge on the dielectric window. As illustrated by the full line in thegraph of FIGURE 4, for conventional uncoated or untreatedradio-fraquency windows, the critical field strength required toaccelerate an electron sufiiciently to commence single surfacemultipactoring and heating as a result thereof corresponds to bombardingelectron velocities ranging between 40 and volts.

After considerable experimentation and analysis, it has been determinedthat by providing a coating or layer of some material on the insidesurface of the dielectric material and adjacent metallic parts, composedof a material having a low secondary emission characteristic, beneficialresults can be obtained in that the value of the critical field strengthrequired to accelerate electrons to a bombarding velocity sufficient tocommence single surface multipactoring is greatly increased as shown bythe dash line in the graph of FIGURE 4. At this higher critical fieldstrength electrons starting from rest are accelerated (2rt+l) halfcycles to a bombarding velocity more than sufficient to release onesecondary electron per bombarding electron corresponding to acoeflicient of secondary electron emission of unity on impact with aconventional window. As seen from the graph, the critical field strengthhas been increased so that it approaches the critical field strength atwhich maximum secondary emission occurs, with maximum secondary emissionoccurring at about or not substantially above unity, as shown. Therealization of these beneficial results depended on the determination ofa material to put on the dielectric and metal surfaces which would nothave deleterious effects either on the electromagnetic transparency ofthe window or its dielectric loss, its chemical stability, itsresistance to thermal stress, and which would not have the tendency offorming an oxide coating on interior metallic surfaces. Other importantfactors also had to be considered, such as the ability of the coating towithstand bakeout temperatures and to be uncontaminating to otherassociated parts.

In addition to the problem of determining what coating material to use,a secondary problem but of equal importance was the problem ofdetermining the thickness of the coating or film and the method of itsapplication. To be effective as an electromagnetic window, thedielectric material obviously must remain transparent to electromagneticenergy, and the interior surface of the window must therefore benonconductive, or if conductive in some small degree, the dielectricloss of the material must remain within certain limits. Another problemwas the method of controlling the thickness of the coating material sothat the effects would be reproducible from one body to another, andfrom one electron tube to another. The method of application of such acoating, and the method of determining the thickness, has been describedin a copending application.

It has been found that a body of dielectric material normally having inthe presence of free electrons and a radiofrequency alternating electricfield of sufficient intensity to accelerate the free electrons to acritical bombardment velocity where the coefficient of secondaryemission of such electrons is greater than unity, may be transformedinto a body of dielectric material having a coeificient of secondaryemission of about or less than unit for a greatly expanded range ofelectric field intensities or bombarding velocities by application toone or all of its surfaces of an extremely thin coating or layer oftitanium. This transformation is illustrated graphically in FIGURE 4which at V indicates the usual critical field strength for conventionaluncoated windows, and at V indicates the expanded range of fieldstrength required before a critical value of bombardment velocity isreached. Titanium may be applied as a coating or layer on the windowafter its manufacture and formation into the requisite configuration,and it is believed the titanium may be incorporated in a surface zone ofthe dielectric material during the process of manufacture of specificwindow configurations from the dielectric material. In either case it isdesirable that the coating or layer of titanium be of reproduciblethickness, and that the'electrical resistance of the dielectric materialremain in the range between to 10 ohms per square. It has been foundthat in order to maintain resistance of a dielectric body within therequisite range, a film is required which is so thin as to be invisibleeither on an opaque dielectric surface or on a transparent dielectricsurface.

The secondary electron emission yield of an electrically conductingsurface is determined by the composition of its surface to the depth ofpenetration of a bombarding pri mary electron. In a radio-frequency(R-F) device, the penetration depth is not very great since the velocityof electrons traveling in an R-F field is limited to that velocity whichcan be attained in one-half of one R-F cycle. In many practical devices,this does not exceed a few hundred volts at a dielectric surface and afew thousand volts in the higher voltage regions of a klystron ortraveling wave tube, for instance. Accordingly, it has been found thatfilms on the order of 1,000 Angstrom units of a dense material such astitanium are sufficient to diminish to about or less than unity thesecondary emission yield in the highest voltage regions of anelectrically conducting surface, and that films Angstrom units or lessare adequate on dielectric surfaces to diminish the secondary emissionyield to about or less than unity. In the case of the dielectricsurface, however, the coating must be discontinuous in nature in orderthat the electrical resistance remain between 10 and 10 ohms per square.Such a discontinuous nature in the coating provides the high resistancewhile also providing the thickness required to control the secondaryelectron emitting characteristics of the surface.

In FIGURE 2 is illustrated the internal resonant cavity portion 2 fromthe external resonant cavity type klystron illustrated in FIGURE 1,which includes an electron gun section 4, a radio-frequency interactionsection 6, and a collector section 7. As is well known in the art, theelectron gun section, the radio-frequency section, and the collectorsection are hermetically united in axial alignment to enable theprojection of an electron beam through a series of drift tube sections8, each terminating within a cavity in a conically tapered end portion 9spaced from the associated end of an adjacent drift tube section toprovide an interaction gap 12 therebetween. Drift tube sections 8 aresupported in axially spaced alignment by relatively heavy transverselyextending annular metallic plates 13, preferably fabricated fromoxygen-free high-conductivity copper. These plates, in turn, are held inaxially spaced relationship by the cylindrical dielectric radiofrequencywindows 14, one of which is designated as the output window. Theopposite ends of the cylindrical dielectric windows are hermeticallyunited to the associated drift tube supporting plates by sealing flangestructures 16 described at greater length in United States Patent2,903,- 614. Such a sealing structure introduces flexibility in thewindow design to accommodate differences in expansion and contraction ofthe dielectric window and the metallic drift tube and supporting plateassembly.

-As illustrated in FIGURE 2, the inner surface 17 of the cylindricaldielectric window is provided with a thin coating or layer -18 oftitanium, deposited thereon by the method described and claimed in acopending application. The coating is preferably deposited by vapordeposition of pure titanium metal in vacuum in a manner to provide auniformly thick coating of titanium over the inner surface of thedielectric window. To complete the cavity, the adjacent metallicsurfaces 19 of the drift tube supporting plates, the conical metallicsurfaces 21 of the drift tube tips within the cavity, and a portion ofthe surface 22 forming the inner bore of each drift tube tip, are coatedwith a layer 23 of pure titanium metal to a thickness of about one-tenthof a mil. In FIGURE 2 the layer of titanium on these surfaces isexaggerated in thickness for clarity.

Experience has taught that a layer of titanium on these surfacessomewhat less than one-tenth of a mil thick is satisfactory and willreduce the secondary electron emission coefiicient of these metal partsto a value of about or less than unity for values of electric fields atwhich other characteristics, such as external arc-over, become limtingfactors. Because coatings of such scant thicknesses are difiicult tomeasure, it has been found that when the characteristicallycopper-colored parts are completely opaque to visual inspection due tothe deposition thereon of pure titanium, the coated metallic parts willappear grey and the thickness of the titanium will be adequate toprovide the requisite low secondary electron emission characteristic. Itwill of course be understood that where desirable, any or all windows ofthe klystron, in addition to other dielectric bodies, such as theelectron gun ceramics, may be coated with titanium with beneficialresults. It should also be understood that the metallic surfaces withinthe evacuated envelope of an integral cavity klystron, including awaveguide output having a 7 fiat output Window, may be coated asdescribed with beneficial results.

In FIGURE 3 is illustrated a fiat circular dielectric waveguide window26 having its periphery 27 hermetically united to the inner surface 28of a metallic waveguide 29. One surface of the window is provided with alayer 31 of titanium about 100 Angstrom units thick, while the adjacentinner metallic surface of the waveguide is provided with a coating 32 oftitanium at least 1,000 Angstrom units thick, or in the alternative,about one-tenth of a mil.

Regarding choice of materials, it is preferred that the metallicelements of the combination be fabricated from oxygen-freehigh-conductivity copper, and that the dielectric materials'be selectedfrom the group including fused silica, alumina-silicate glass, steatite,forsterite, alumina, beryllia or pyroceram. Materials selected from thisgroup will be found to have a dielectric constant ranging from about 3.8for fused silica to about 8 or 9 for alumina. In terms of the powerfactor or loss tangent, it will be found that a selection may be madefrom this group in which the value of this characteristic is in therange between .0001 for fused silica to about .0008 for beryllia forfrequencies of about 10 c.p.s. The thermal coeflicients of expansion ofthese materials range between about 10- for fused silica to about l 10"'for forsterite. The moduluses of elasticity of the selected materialsrange between a low of 10 10 for fused silica to a high of about 40 l0-for alumina. Of these six materials, the three that are preferred formicrowave applications are fused silica, alumina and beryllia.

In conclusion it may be stated that any radio-frequency window of anyconfiguration will be heated to some extent by various causes. It isimportant to conduct away as much of such heat as possible. regardlessof the cause. It has been found that windows fabricated from beryllia,coated with titanium, and incorporated in metallic structures fabricatedfrom oxygen-free, high conductivity copper which are also coated withtitanium provide an almost ideal window package.

We claim:

1. In a radio-frequency device including an evacuated section havingjuxtaposed metallic and dielectric portions with adjacent metallic anddielectric surfaces, said surfaces being exposed to vacuum within theevacuated section, means on said adjacent metallic and dielectricsurfaces within the evacuated section providing a maximum coeflicient ofsecondary electron emission ranging downwardly from about unity, saiddielectric surface with said means thereon providing dielectricpropetries in said device.

2. The combination according to claim 1, in which said radio-frequencydevice comprises a metallic waveguide and a dielectric radio-frequencyoutput window assembly, at least one surface of said window and theadjacent associated surface of the waveguide being exposed to vacuumwithin said evacuated section.

3. A radio-frequency device comprising an evacuated envelope havingjuxtaposed metallic and dielectric portions with metallic and dielectricsurfaces exposed to vacuum, incident electrons, and electromagneticenergy within said envelope, said dielectric surface having meansthereon providing a maximum coefficient of secondary electron emissionranging downwardly from about unity, said dielectric surface with saidmeans thereon providing dielectric properties in said device.

4. A radio-frequency device according to claim 3 wherein said dielectricportion is an output window through which said electromagnetic energypasses.

References Cited UNITED STATES PATENTS 3,059,142 10/1962 Vaughan 333-982,821,659 1/1958 Feinstein 315-3977 2,955,229 10/1960 Bondley 313-107 X2,958,804 11/1960 Badger et al 315-539 HERMAN KARL SAALBACH, PrimaryExaminer. SAXFI-ELD CHATMOH, JR., Assistant Examiner.

US. Cl. X.R.

